Porous plugs are advantageously used in vessels for molten metal, such as ladles, ladle refining furnaces and tundishes, in order to render uniform the temperature of the molten metal and to accelerate desired refining reactions. Typically, the porous plug includes a porous refractory body which may have assembled thereon a steel or iron jacket which is connected to a gas inlet pipe. The porous plug is fitted in an opening in the bottom of the vessel and a gas such as nitrogen or argon is blown through the porous plug into the molten metal contained in the vessel. The gas bubbles up from the exposed top surface of the porous refractory body of the plug through the molten metal contained in the vessel to effect stirring and mixing of the molten metal and to obtain other advantageous results.
In use, the porous plugs are subject to erosion and melt-down at their top surfaces. Usually, melt-down of a porous plug progresses at a rate faster and more indeterminate than that of the refractory lining of the vessel for the molten metal. Therefore, to prevent leakage of the molten metal through the plug-mounting opening or port, which may be quite catastrophic, the remaining life of the plug must be ascertained after each use of the vessel and preferably without having to first cool the vessel in order to determine whether the plug has served its useful life and should be replaced. With molten metals such as steel, plug melt-down is particularly severe because the plug is subject to high pressure, turbulent contact with the molten steel, such as at 1870.degree. K. or higher. Such severe melt-down, or actually breakage, results from the invasion or penetration of molten steel into the exposed surface layer of the plug, such being caused by the heavy pressure or weight of the molten steel acting on the plug when blowing of the gas through the plug has ceased. In addition, erosion of the plug is causd by slag acting thereagainst after the molten metal has been poured out of or otherwise removed from the vessel.
Heretofore, the remaining useful life of a porous plug has been ascertainable in the following ways:
(a) A bar may be inserted through the upper end of the vessel to measure the remaining height of the porous plug, such measurement being made relative to the bottom surface of the vessel.
(b) For porous plugs which vary in cross-section along their vertical axes, the diameter of the eroded top surface of the porous plug may be measured at a small distance therefrom by using an optical measuring instrument, thereby to estimate the remaining height of the plug.
(c) For porous plugs provided with discontinuously varying diameters, the remaining life of the plug may be ascertained by measuring the then exposed diameter.
All these methods of measuring the life of the porous plugs, however, are disadvantageous for one or more reasons. For instance, method (a) is dangerous when the measurements are taken while the vessel is at a high temperature. Moreover, measuring error may occur because of the unevenness of the base surface. Method (b) may be better but in addition to the difficult problem of maintenance of the precision optical measuring instrument under extreme environmental conditions of rapidly changing temperatures, measuring error occurs because of the visual distortion caused by convection of hot air. As a result, it is necessary to replace the porous plug prematurely to account for the measuring error and thus to assure that the plug is replaced before it wears out. Such premature replacement of the plug is uneconomical and alters or disrupts the use plan of the vessel. As for method (c), the observer must look for the discontinuous change of diameter in the direction of the larger diameter in view of the relation of the plug diameter to gas-flow resistance. Since the experimentally determined, safe, remaining height of a plug is 100 mm. or one half of the diameter of the bottom surface of the plug, method (c) disadvantageously requires excess, expensive porous refractory material to be used.